Recently, wireless LANs (Local Area Networks) have been used in offices and homes and for hot spot services and the like in public places and have been rapidly spreading. The mainstream of the current wireless LAN standards is IEEE802.11a using 5 GHz band and IEEE802.11b/g using 2.4 GHz band. Furthermore, IEEE802.11e in which the MAC (Medium Access Control) layer stipulated in IEEE802.11a/b/g is extended and the QoS (Quality of Service) function is added has also been established as a standard. Thus, wireless LAN standardization activities are actively promoted.
Currently, standardization activities for IEEE802.11n for extending the physical/MAC layers, aiming at practically achieving throughput above 100 Mbps are conducted, and furthermore, examination of GigaLAN aiming at realization of a transmission rate of G (Giga) bps has been started.
As one of approaches for improving throughput, IEEE802.11n proposes a method in which multiple frequency channels are used at the same time to transmit wireless signals (for example, Non-patent Document 1: IEEE802.11n Working Group, “Draft Amendment to STANDARD [FOR] Information Technology-Telecommunications and information exchange between systems—Local and Metropolitan networks—Specific requirements—Part 11: Wireless LAN Medium Access Control and Physical Layer specifications: Enhancements for Higher Throughput,” IEEE P802.11n™/D1.06, November 2006).
Non-patent Document 1 described above, therein disclosed a technique in which two frequency channels with a bandwidth of 20 MHz used in an IEEE802.11 wireless LAN are used at the same time to realize wireless communication with a bandwidth of 40 MHz.
When multiple frequency channels are used at the same time to transmit wireless signals, such as in the case where the frequency channel used by an own BSS (Basic Service Set) and the frequency channel used by an OBSS (Overlapping Basic Service Set) overlap with each other, the wireless signals cannot be transmitted with the use of the multiple frequency channels unless it is confirmed that the carrier sense results of all the frequency channels to be used are “idle”. Therefore, for example, in the case where any of the multiple frequency channels to be used is busy, the wireless base station has to wait until all the frequency channels to be used become idle. Even if broadbanding of frequency channels is realized by using multiple frequency channels at the same time to transmit wireless signals, there is a problem that the waiting time before starting transmission of the wireless signals is increased, and the practical throughput decreases.
In addition to Non-patent Document 1 described above, there is also disclosed a technique in which carrier sense is performed for multiple frequency channels before transmission of wireless signals, and the wireless signals are transmitted with the use of only frequency channels recognized to be idle, as one of techniques for transmitting wireless signals using multiple frequency channels at the same time to improve throughput (for example, Non-patent Document 2: WWiSE, “WWISE Proposal: High throughput extension to the 802.11 Standard,” WWiSE Draft, August 2004).
Non-patent Document 2 described above has the following problem. Though the waiting time before starting transmission of the wireless signals does not increase, the number of frequency channels used at the same time to transmit the wireless signals decreases and the bandwidth of the frequency channels may be narrow because frequency channels recognized to be busy as a result of the carrier sense performed just before the transmission are not used. Therefore, it is difficult to improve the practical throughput.